This rule guarantees that a subprocess stays clear and comprehensive.A subprocess should have exactly one Start Event. If a subprocess has more than one Start Event, the trigger might become unclear.
The clear definition of one Start Event is important because it is defined as the subprocess’ trigger. When using multiple start events, it may not be clear when a process is to begin. There should only be one Start Event in a BPMN subprocess. If this rule is broken, make sure all event types are used correctly in the diagram.